1. Field of the Invention
This invention resides in the field of steel alloys, particularly those of high strength, toughness, corrosion resistance, and cold formability, and also in the technology of the processing of steel alloys to form microstructures that provide the steel with particular physical and chemical properties.
2. Description of the Prior Art
Steel alloys of high strength and toughness and cold formability whose microstructures are composites of martensite and austenite phases are disclosed in the following United States patents (all assigned to The Regents of the University of California), each of which is incorporated herein by reference in its entirety:
U.S. Pat. No. 4,170;497 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Aug. 24, 1977 PA1 U.S. Pat. No. 4,170,499 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Sep. 14, 1978 as a continuation-in-part of the above application filed on Aug. 24, 1977 PA1 U.S. Pat. No. 4,619,714 (Gareth Thomas, Jae-Hwan Ahn, and Nack-Joon Kim), issued Oct. 28, 1986 on an application filed Nov. 29, 1984, as a continuation-in-part of an application filed on Aug. 6, 1984 PA1 U.S. Pat. No. 4,671,827 (Gareth Thomas, Nack J. Kim, and Ramamoorthy Ramesh), issued Jun. 9, 1987 on an application filed on Oct. 11, 1985
The microstructure plays a key role in establishing the properties of a particular steel alloy, and thus strength and toughness of the alloy depend not only on the selection and amounts of the alloying elements, but also on the crystalline phases present and their arrangement. Alloys intended for use in certain environments require higher strength and toughness, and in general a combination of properties that are often in conflict, since certain alloying elements that contribute to one property may detract from another.
The alloys disclosed in the patents listed above are carbon steel alloys that have microstructures consisting of laths of martensite alternating with thin films of austenite and dispersed with fine grains of carbides produced by autotempering. The arrangement in which laths of one phase are separated by thin films of the other is referred to as a "dislocated lath" structure, and is formed by first heating the alloy into the austenite range, then cooling the alloy below a phase transition temperature into a range in which austenite transforms to martensite, accompanied by rolling to achieve the desired shape of the product and to refine the alternating lath and thin film arrangement. This microstructure is preferable to the alternative of a twinned martensite structure, since the lath structure has a greater toughness. The patents also disclose that excess carbon in the lath regions precipitates during the cooling process to form cementite (iron carbide, Fe.sub.3 C) by a phenomenon known as "autotempering." These autotempered carbides are believed to contribute to the toughness of the steel.
The dislocated lath structure produces a high-strength steel that is both tough and ductile, qualities that are needed for resistance to crack propagation and for sufficient formability to permit the successful fabrication of engineering components from the steel. Controlling the martensite phase to achieve a dislocated lath structure rather than a twinned structure is one of the most effective means of achieving the necessary levels of strength and toughness, while the thin films of retained austenite contribute the qualities of ductility and formability. Achieving this dislocated lath microstructure rather than the less desirable twinned structure requires a careful selection of the alloy composition, since the alloy composition affects the martensite start temperature, commonly referred to as M.sub.s, which is the temperature at which the martensite phase first begins to form. The martensite transition temperature is one of the factors that determine whether a twinned structure or a dislocated lath structure will be formed during the phase transition.
In many applications, the ability to resist corrosion is highly important to the success of the steel component. This is particularly true in steel-reinforced concrete in view of the porosity of concrete, and in steel that is used in moist environments in general. In view of the ever-present concerns about corrosion, there is a continuing effort to develop steel alloys with improved corrosion resistance. These and other matters in regard to the production of steel of high strength and toughness that is also resistant to corrosion are addressed by the present invention.